The objective of this study is the classification of heterogeneous leukemic lymphocytes by specific biochemical, biophysical and immunological tests. The drug resistant cells and cells in GO state of the cell cycle will be separated and characterized by multiple parameters. To carry out these goals we have initiated the study of the cell populations in the normal AKR mouse thymus, spontaneous and first generation transplanted AKR leukemia, and L1210 Ascites tumor systems. The study of the normal thymus development revealed three populations of thymocytes. The smallest cell population was most sensitive to radiation, cyclophosphamide, and hydrocortisone. Both radiation and cyclophosphamide selectively killed small thymocytes, and stimulated a production of medium and large lymphocytes. The hydrocortisone resistant population, comprising less than 5% of the thymus, was shown to be lighter and larger than most of the thymocytes. This population was also heterogeneous in size. This data on size and density of normal thymus served as a foundation for the study of AKR spontaneous leukemia and the reaction of AKR leukemic thymus chemotherapy. Thymus from AKR spontaneous leukemia animals were compared for thymus weight, labeling index, cell size, spleen weight, and lymph node size. The data using these parameters could not be effectively correlated by statistical analysis. In leukemic animals, the modal population size of thymocytes ranged from 125 cubic microns to 400 cubic microns and the corresponding thymus weight ranged from 50 milligrams to 1000 milligrams. Biomodal size distributions were also observed. The normal animal had a modal size of 104 cubic microns and a weight of 80 milligrams.